Rapamycin is a bacterially derived natural product that has potent anti-proliferative effects. A special formulation of rapamycin, called CCI-779, is under large-scale phase 3 clinical trials for renal cell cancer and phase 2 clinical trials for breast cancer. It is also under early stage clinical trials for many other cancers, including colon, prostate and brain cancers. During these trials, CCI-779 is well tolerated and shows significant anti-cancer responses. In addition to directly inhibiting cancer cells, rapamycin blocks VEGF-mediated endothelial cell proliferation, thereby hindering tumor growth by blocking tumor angiogenesis. It is anticipated that CCI-779 will become a major new cancer drug. However, like many other cancer drugs, treatment with CCI-779 has risks of side effects. Moreover, some cancer cells become highly resistant to rapamycin, the mechanism of which is poorly understood. Therefore, it is important to study the molecular mechanism of rapamycin action, and the genetic factors that determine the sensitivity of cancer cells to rapamycin. Rapamycin inhibits cellular proliferation by interfering with the functions of the target of rapamycin protein (TOR). TOR is a central controller of cellular growth and proliferation. The TOR proteins and TOR signaling pathways are highly conserved between humans and yeast. Genetic and biochemical evidence has clearly established that genetic mutations in the components of TOR pathways can significantly affect rapamycin sensitivity of both yeast and cancer cells. We have recently conducted a genomic rapamycin sensitivity screen using the systematically generated yeast deletion mutants and identified the yeast rapamycin-sensitive genes and genetic pathways. In this proposal, we will study two major conserved rapamycin-sensitive pathways that may be excellent new targets for anti-cancer drug discovery. Small chemical ligands electively disrupting one or more of these pathways may prove to be highly specific against cancer proliferation, since they do not interfere with other TOR pathways as does rapamycin. In addition, they may be used to sensitize cancer cells to rapamycin by mimicking the genetic mutations in their target pathways. Therefore, these chemical ligands should have fewer side effects than rapamycin. Moreover, these studies will lead to better understanding of the mechanisms of rapamycin action and of genetic factors governing rapamycin sensitivity of cancer cells.